The construction of semiconductor devices involves using controlled vapors to apply very thin metal coatings to silicon wafers. Chemical vapor deposition (CVD) is a process for applying these coatings. In this process, a volatile metal-organic precursor gas comes in contact with areas of a wafer where a metal film is needed. A chemical reaction occurs, resulting in the deposition of metal on the surface. The coatings, however, must be applied only to specific parts of the wafer. If the coating is deposited on the bevel or edge of the wafer, it may cause the coating on the frontside of the wafer to peel away. If the coating attaches to the backside of the wafer, it may contaminate other pieces of equipment as the wafer undergoes further processing. Finally, the precursor gas may reach areas of the equipment under the backside of the wafer, causing contamination of the process equipment.
Known methods for preventing a precursor gas from reaching the back parts of a wafer, or from contaminating the equipment include contact-based methods and purge-based methods. The contact-based methods use a ring to shield the bevel, edge, and backside of the wafer. The ring comes in contact with the front surface of the wafer in order to prevent the reactive gas from reaching the lower parts of the wafer. The contact methods, however, are problematic because contact with the wafer may cause particle generation. Moreover, the ring covers part of the wafer, reducing the area on which a coating may be deposited. In addition, since the ring is cooler than the wafer, it cools part of the wafer surface, causing a drop in the deposition rate, which leads to irregularities in the coating. In the purge-based methods, a ring partially shields the lower parts of the wafer without touching the wafer, leaving a gap between the ring and the wafer. An air flow of an inert gas is directed from under the shield through the gap to prevent the precursor gas from reaching the lower parts of the wafer. A problem with the purge methods is that the gas flow needed to prevent the precursor gas from entering the gap causes deformities in the metal coating.
While these devices and methods have provided improvements over prior approaches, the challenges in the field of circuit fabrication has continued to increase with demands for more and better techniques having greater effectiveness. Therefore, a need has arisen for a new method and system for constructing semiconductor devices.